Pattern and method of making same



United States Patent ice 3,149,382 PATTERN AND METHOD OF MAKING SAME Chauncey A. Hoag, Clarernont, Caiifi, assignor to Mertronics Corporation, Santa Monica, Calif., a corporation of Delaware No Drawing. Filed Dec. 18, 1961, Ser. No. 160,243 14 Claims. (Cl. 22-164) This invention relates generally to the art of preparing mold patterns used in investment casting, and more particularly concerns the preparation of pattern sections for secure booking, such preparation involving treating section surfaces with certain selected flux agents.

In my companion application entitled Investment Casting Alloy Composition and Method, there is described a novel investment casting alloy composition comprising mercury, thallium and a metallic substance such as zinc which is electro-positive to thallium. The zinc is present in dissolved state in the alloy and in amount sufiicient to inhibit oxidation of the thallium. Since the zinc oxidizes in preference to thallium the dissolved zinc content of the alloy protects the thallium against oxidation so long as dissolved zinc metal is present.

Typically, processing and use of the alloy includes the steps of the investment of dies with liquid alloy, freezing of the alloy in the dies, stripping the dies from the frozen alloy pattern sections, fluxing and booking the patterns sections to form a complete pattern, forming ceramic molds on the frozen alloy patterns, melting the frozen alloy patterns to free the molds for commercial foundry use, separating impurities including extraneous and suspended oxides from the melted alloy, replenishing depleted zinc to the alloy, storing and mixing the fluid alloy for reuse.

As described in my companion application, the thallium content of the alloy is between 28.2 and 31.8 percent by weight, and the alloy contains less than 3.0 percent zinc by weight, and typically is close to 0.6 percent, these percentages having been found to result in the overcoming of certain problems such as rough surfaces on resulting castings and inferior booking properties of the alloy pattern sections.

The present invention is directed to a further refinement or improvement in the method of preparing mold patterns, which are formed of a liquid alloy composition comprising mercury, thallium and a metallic substance such as zinc which is electro-positive to thallium, the alloy being frozen to form mold patterns to be booked. Specifically, the improvement has to do with treating of at least one and preferably both of the pattern complementary booking surfaces with a fiuxing agent characterized as having the properties of markedly facilitating the booking of the frozen pattern section complementary surfaces, particularly patterns having the thallium and zinc compositions mentioned above. As will be described, the fluxing agent is selected from the group that includes acetic acid and aqueous solutions of ammonium acetate, zinc acetate and thallium acetate.

The problem involved in making secure booking or joining of complementary frozen patterns of this alloy is considered to be essentially one of prompt removal of surface zinc oxide with little or no etching of the frozen alloy proper, by a solution of low surface tension that will remain free of solids at the temperature of booking.

3,149,382 Patented Sept. 22, 1964 Most common mineral acids and some organic acids are too highly ionized and etch the frozen alloy too deeply. Acetic acid on the other hand is moderately ionized, and is readily soluble in Water and its salts are readily soluble in water. While some other acids might be used as a basis for the formulation of a suitable flux solution, acetic acid is considered ideal because it is commercially available and well adapted to the intended use. After the patterns are frozen, they are likely to have surface films of metallic oxide and condensed moisture, freezing being typically carried out at temperatures around 35 degrees F. or less. A flux to permit actual contact of the frozen alloy pattern surfaces must dissolve the oxide film and absorb the condensed moisture, with the further properties of being sufliciently concentrated to resist freezing at the temperature of the frozen patterns and to have exceptionally low surface tension. It was found that a flux selected from the group that includes acetic acid, ammonium acetate and acetate salts of zinc and thallium in aqueous solution has the desired properties in that the application of such a flux to the surfaces to be booked results in exceptionally good booking of the test sections. These acetate fluxes are characterized as being neutral to weakly acidic, so that they do not deeply etch or discolor the surfaces to be booked. Their low surface tension properties permit them relatively easy movement from crevices in the surfaces of the patterns, facilitating good booking.

In the following described tests, duplicate alloy portions of about 40 grams each were placed in separate Petri dishes, submerged in liquid, then frozen. Most of the test pats were frozen at about 5 degrees F., submerged in acetone. The frozen pats, removed from the acetone and placed in dry chilled Petri dishes were moistened with the solution being tested as flux, and then they were stacked one on the other and left there for about a minute and then tested. In general, the application of a slight pressure to the top test piece was needed to place the contacting surfaces in intimate contact suflicient to produce secure booking. If more than a very slight pressure was required to bring the test pieces together close enough to adhere, the weld was usually poor.

Booking Designations Secure booking: Meaning a strong weld, so strong that in prying the test pats apart with a strong steel spatula the frozen metal was badly deformed and the metal at the weld was torn roughly, and deeply.

Good booking: Meaning a weld strong enough to resist separation by prying with a strong steel spatula until the frozen metal showed moderate deformation before separating at the Weld, and leaving moderately rough spots Where the metal had been welded.

Poor booking: Meaning a weak weld that could be pried apart readily without visible distortion of the frozen test pats and leaving only slight roughness where the pats had been joined.

No booking: Meaning the frozen metal pats were so readily separated as to appear to be useless for the purpose intended.

A series of booking tests was made to determine whether flux solution should be applied to one or to both of the surfaces to be joined, before the parts were brought into contact.

The above comparative tests were made with frozen pats made from an alloy batch containing 0.6 percent zinc, 38.8 percent thallium and 60.6 percent mercury.

Solution ot- Flux put on Booking result Thallium acetate Both p Secure booking.

Do One pat only Poor booking.

'do Good booking. do Poor booking.

Comparative Booking Tests With Salts of Acetic Acid The following solutions were made up for use as fluxes for joining frozen pats of zinc-thallium-mercury alloys. The purpose was to determine which worked best as a flux, on alloy containing about 0.7 percent zinc, 30.0 percent thallium and 69.3 percent mercury.

Solution marked Material Concentration of water 40 solution used A Zinc acetate, C.P Saturated, at room temperature. B- Ammonium acetate, Do.

O.P. crystals.

Thallium acetate, Do.

prepared. Mercuric acetate, Do.

0.1. powder. E Acetic acid, o.p. 99.8% (No water glacial. added).

The solution C thallium acetate was prepared by long digestion of a portion of thallium metal in acetic acid, under a reflux condenser, after which the fluid was transferred to an evaporating dish, placed in a warm place until dry, then dissolved in a small amount of water. The solution was saturated at room temperature, as shown by the presence of crystals with the solution. This solution is slightly acid in chemical reaction, showing a reddish orange coloration when treated with methyl red indicator, indicating pH 4.5 approximately.

In the booking tests itemized below the solutions were applied dropwise by means of a medicine dropper and application was made to both of the frozen parts to be joined. The joined test pats were allowed to stand about one minute before they were tested, held in chilled Petri dishes.

Booking test result, remarks C Booked securely. D No booking. Mercury film left on test pats. E No booking. Frozen pats corroded.

It seems clear that solutions of ammonium. acetate and 7 i.- of thallium acetate serve satisfactorily as single salt fluxes for use on these zinc-thallium-mercury alloys.

Two solutions combined in equal volume, as flux: Booking test result AB (Zn-l-amm.) Good booking.

AC (Zn-l-Tl) Secure booking.

AD (Zn-i-Hg) Poor booking.

AE (Zn+acetic acid) Secure booking.

BC (amm.+Tl) Secure booking.

BD (amm.+Hg) Poor booking.

BE (amm.+ac. a.) Secure booking.

CD (TH-Hg) Poor booking.

CE (Tl-l-ac. a.) Poor booking (too much corrosion).

DE (Hg+ac. a.) Poor booking (Hg film soft).

In the above series of comparative booking tests it may be noted that among the combinations that included zinc acetate, only two gave secure booking, and that these two combinations included acetic acid and thallium acetate solution with a slight acid reaction. The ammonium acetate added to the zinc acetate gave better booking than the zinc acetate solution alone, but the mercuric acetate addition to the zinc acetate solution did not improve the booking.

Among the remaining combinations that included ammonium acetate, secure booking was secured with combinations including thallium acetate and acetic acid. Among the combinations that included thallium acetate, secure booking was secured with zinc acetate and ammonium acetate, but not with mercuric acetate or with acetic acid. Among the combinations that contained mercuric acetate, not one gave secure booking. Among the combinations that contained acetic acid, secure booking was obtained with zinc acetate and with ammonium acetate only.

It seems clear that mercuric acetate is not a desirable component of a flux for these alloys, and that there is a definite limit on the proportion of acetic acid to be used in such a flux. It also seems clear that ammonium acetate is the equal of thallium acetate and the superior of zinc acetate as a component of any two salt flux for these alloys.

Three solutions combined in equal volume for use Booking test as flux: result ABC (Zn+a.mm.+Tl) Secure booking. ABD (Zn-|-amm.+Hg) Poor booking. ABE (Zn+amm.+ac. a.) Secure booking. ACD (Zn+Tl+Hg) Poor booking. ACE (Zn+Tl+ac. a.) Secure booking. ADE (Zn-l-Hg-J-ac. a.) Poor booking. BCD (amm.+Tl+Hg) Poor booking. BCE (amm.+Tl-l-ac. a.) Secure booking. BDE (amm.+Hg+ac. a.) Good booking. CDE (Tl+Hg+ac. a.) Good booking.

In the above series of three component fluxes, only mercuric acetate was absent from the four that produced secure booking. In the four fluxes that produced secure booking, zinc was present in three, ammonium acetate was present in three, thallium acetate was present in three and acetic acid was present in three.

In the two fluxes that rated Good, mercury was present in both, acetic acid was present in both and thallium acetate was present in one and ammonium acetate was present in one. On this basis the ammonium acetate and the thallium acetate were about equally effective in suppressing the adverse eflect of the mercury. Where zinc acetate was combined with mercuric acetate and acetic acid, the flux produced poor booking, so that zinc acetate should be rated below thallium acetate and ammonium acetate, in usefulness as a flux component.

From the foregoing tests it was found that the booking of frozen zinc-thallium-mercury alloys requires that both surfaces to be joined should be moistened with the flux solution, and that the flux should be applied only a few seconds before the frozen parts are pressed into intimate contact. Single component fluxes that proved to be eifective were concentrated aqueous neutral solution of ammonium acetate and concentrated aqueous acid solution (at pH 4.5) of thallium acetate. The double component fluxes that proved to be effective were as follows:

(a) Aqueous solution of ammonium acetate, and acetic acid,

(b) Aqueous solution of ammonium acetate and thallium acetate,

Aqueous solution of thallium acetate and zinc acetate,

(d) Aqueous solution of zinc acetate and acetic acid.

The triple component fluxes that proved to be effective were as follows:

(a) Aqueous solution of ammonium acetate, aqueous solution of zinc acetate, and aqueous solution of thallium acetate,

(b) Aqueous solution of ammonium acetate, aqueous solution of thallium acetate, and acetic acid,

(0) Aqueous solution of ammonium acetate, aqueous solution of zinc acetate, and acetic acid,

(d) Aqueous solution of zinc acetate, aqueous solution of thallium acetate, and acetic acid.

In these tests of the double component and triple component fluxes the components of the selected group were present in approximately equal concentration, resulting from mixing of equal volumes of the aqueous solution components having the same concentration.

It was found that mercuric acetate solution alone or in any combination with the other acetate salts, decreased the strength of the weld by interposing a film of mercury metal. Also, acetic acid alone or in large proportion in any mixture with an acetate salt was too rapidly corrosive, but when added in moderation was helpful. Zinc acetate solution alone did not work well as a flux, but did work well when combined with thallium acetate or with acetic acid or when combined with either of these and ammonium acetate. The most satisfactory components of the most effective fluxes tried were ammonium acetate and thallium acetate, whereas the addition of zinc acetate to either or both of these in aqueous solution is considered optional. Additions of acetic acid to the fluxes should be carefully limited so as to avoid excessive corrosion of the frozen alloy. When the solution was adjusted to have a chemical reaction between pH 7.0 and pH 4.0, and preferably pH about 4.5, it was entirely satisfactory and sufiiciently acid. Adjustments with acetic acid to pH 4.5 can be made with the aid of methyl red indicator solution by stopping the acid addition as soon as a reddish orange color is reached.

It was further found that the concentration of flux solution should be regulated by the temperature of the frozen patterns at the time they are to be joined, the solution concentration increasing for lower temperature patterns. More specifically, a to percent solution of the flux is strong enough if the patterns are at 35 degrees F., but a 55 percent solution of the flux is needed if the frozen patterns are at a temperature of 5 degrees F. when booking is carried out.

Further steps of the process including leaving the patterns undisturbed for about a minute after booking in order to allow the weld to book securely. After booking, the patterns should be contacted with chilled water, as by submerging and storing the patterns in ice water, in order to remove excess flux solution from the patterns as by diffusion. Such removal of excess flux solution is considered necessary in order to prepare the patterns for molding by application of the ceramic spray previously mentioned. Just prior to such molding, the patterns are dried as by successive rinsing in acetone.

I claim:

1. The method of preparing mold patterns that includes forming a liquid alloy of mercury, thallium and a metallic substance which is electro-positive to thallium, freezing said alloy to form mold patterns to be booked, treating at least one of the pattern complementary booking surfaces with liquid fluxing agent selected from the group consisting of acetic acid and aqueous solutions of ammonium acetate, zinc acetate and thallium acetate, and booking said complementary surfaces while the patterns remain frozen with said fluxing agent at approximately pattern temperature and in liquid state.

2. The method of preparing freely booking mold patterns that includes forming a liquid alloy of mercury, thallium and zinc, freezing said alloy to form mold patterns to be booked, treating complementary pattern booking surfaces with a flux solution selected from the group consisting of acetic acid and aqueous solutions of ammonium acetate, zinc acetate and thallium acetate, and booking said complementary surfaces while the patterns remain frozen below 40 F. with said fluxing agent at approximately pattern temperature and in liquid state.

3. The method of claim 2 wherein the alloy zinc is present in amount sufficient to inhibit oxidation of the thallium.

4. The method of claim 3 wherein the alloy contains between 0.0 and 3.0 by weight zinc.

5. The method of claim 4 wherein the alloy contains between 28.2 and 31.8 percent thallium, by weight.

6. The method of claim 2 in which said treating step includes filming the flux on said surfaces.

7. The method of claim 6 in which said booking step is carried out immediately after said filming step.

8. The method of claim 2 in which the concentration of the aqueous solution is within the range 5 percent to 55 percent for treated pattern temperatures ranging between 35 degrees F. and 5 degrees F.

9. The method of claim 2 in which the solution has a chemical reaction between pH 7.0 and pH 4.0.

10. The method of claim 2 in which the flux consists of multiple components from said group present in approximately equal concentration.

11. The method of claim 2 including the step of contacting the booked patterns with chilled water to remove excess flux therefrom.

12. Freely booking mold patterns comprising solid bodies of frozen alloy consisting of mercury, thallium and zinc, said bodies having complementary surfaces to be booked, and a fluxing agent filming said surfaces, said agent being selected from the group consisting of acetic acid and aqueous solution of ammonium acetate, zinc acetate and thallium acetate.

13. The invention as defined in claim 12 in which the chemical reaction of the solution is between pH 7.0 and pH 4.0.

14. The invention as defined in claim 12 in which the solution concentration is within the range 5 percent to 55 percent and the pattern surface temperature is within the range 35 degrees F. and 5 degrees F.

References Cited in the file of this patent UNITED STATES PATENTS 2,122,185 Smith June 28, 1938 2,857,641 Kramer Oct. 28, 1958 2,890,141 MacCormack June 9, 1959 

1. THE METHOD OF PREPARING MOLD PATTERNS THAT INCLUDES FORMING A LIQUID ALLOY OF MERCURY, THALLIUM AND A METALLIC SUBSTANCE WHICH IS ELECTRO-POSITIVE TO THALLIUM, FREEZING SAID ALLOY TO FORM MOLD PATTERSN TO BE BOOKED, TREATING AT LEAST ONE OF THE PATTERN COMPLEMENTARY BOOKING SURFACES WITH LIQUID FLUXING AGENT SELECTED FROM THE GROUP CONSISTING OF ACETIC ACID AND AQUEOUS SOLUTIONS OF AMMONIUM ACETATE, ZINC ACETATE AND THALLIUM ACETATE, AND BOOKING SAID COMPLEMENTARY SURFACES WHILE THE PATTERNS REMAIN FROZEN WITH FLUXING AGENT AT APPROXIMATELY PATTERN TEMPERATURE AND IN LIQUID STATE. 